Services for delivering video content such as movies via the Internet, etc., have become none common in recent years. Video content is encoded by Digital Rights Management (DRM) technology before delivery. Illegal copying or distribution of such video content is prevented due to DRM technology.
However, many incidents have occurred in which video content, illegally copied by capturing the video content displayed on a computer display or a television monitor by using an image capturing apparatus such as a camcorder, has been illegally distributed. Since the video content displayed on the display is not encoded it is difficult to prevent the distribution of the video content captured by video.
In view of the above, a technology has been developed that embeds information such as a viewer identification number into moving image data in the form of a digital watermark before delivering the video data. If the video data in which the digital watermark is embedded is illegally copied by capturing the displayed image using a camcorder, the digital watermark remains intact in the copied video data. Therefore, if the video data illegally copied by means of such analog capture is distributed, the identification number of the viewer that has made illegal use of the video content can be identified by detecting the digital watermark from the illegally copied video data. Accordingly, even when the video content illegally copied by means of analog capture is uploaded to a video uploading website, for example, the administrator of the video content can identify the illegal copy source from the information embedded in the uploaded video content.
It is preferable to embed an digital watermark in video data in such a manner that the digital watermark embedded in the video data is imperceptible to the viewer. To embed the digital watermark in the video data so as to be imperceptible to the viewer, it is preferable to reduce the variation in luminance that occurs due to the presence of the digital watermark, i.e., the difference in pixel value between the digital watermark embedded region and its peripheral region. However, if the difference in pixel value between the digital watermark embedded region and its peripheral region is small, the digital watermark may be lost when the video data in which the digital watermark is embedded is compressed using a standard such as Moving Picture Experts Group (MPEG). The digital watermark may also be lost due to image distortions, etc., that occur when the video data in which the digital watermark is embedded is captured using a camcorder or the like. On the other hand, if the difference in pixel value between the digital watermark embedded region and its peripheral region is large, such a problem does not occur, but in this case, the digital watermark becomes perceptible to the viewer. That is, there is a tradeoff between the degree to which the pixel value can be made to spatially change due to the presence of the digital watermark and the degree to which the digital watermark can be made robust. To solve this tradeoff problem, various digital watermarking technologies have been invented (for example, refer to International Patent Publication No. 2007/102403).
A digital watermark embedding apparatus disclosed in International Patent Publication No. 2007/102403, for example, embeds information as a digital watermark into an input signal having dimensions equal to or greater than N. This digital watermark embedding apparatus generates an (N−1)-dimensional pattern based on the embedding information, generates an N-dimensional embedding pattern by modulating a periodic signal in accordance with values on the (N−1)-dimensional pattern, and superimposes the embedding pattern on the input signal.
However, in the case of the technology disclosed in International Patent Publication No. 2007/102403, the luminance of the entire embedding pattern changes. As a result, if an image with uniform brightness and with little temporal variation, such as the sky, is contained in the region where the embedding pattern is embedded, the moving image in which the digital watermark is embedded may be perceived as if the image were flickering. Furthermore, the boundary between two regions with different embedded values may become noticeable. This can lead to a degradation of the picture quality of the moving image in which the digital watermark is embedded.